U.S. patent application number 10/476684 was filed with the patent office on 2004-12-30 for combined fluidized bed and pulverized coal combustion method.
Invention is credited to Dernjatin, Pauli, Fabritius, Marko, Jaaskelainen, Kari, Savolainen, Kati.
Application Number | 20040261675 10/476684 |
Document ID | / |
Family ID | 8555905 |
Filed Date | 2004-12-30 |
United States Patent
Application |
20040261675 |
Kind Code |
A1 |
Dernjatin, Pauli ; et
al. |
December 30, 2004 |
Combined fluidized bed and pulverized coal combustion method
Abstract
The invention relates to a combined fluidized bed and pulverized
coal combustion method and system. In the method, fluidizing air
(4) is injected into a fluidized bed (2) situated in the bottom
portion of the combustion chamber (3). Into the combustion chamber,
to above the fluidized bed (2), is fed a mixture of pulverized coal
and a carrier gas from a second set of fuel feed means (6) at a
mass flow rate which is higher or at least substantially equal to
the upper ignition limit of the mixture, and the mixture of the
pulverized coal and the carrier gas at least by the fluidizing air
(4), and at least a fraction of the fuel fed via the second set of
fuel feed means (6) is combusted above the fluidized bed (2).
Inventors: |
Dernjatin, Pauli; (Helsinki,
FI) ; Savolainen, Kati; (Vantaa, FI) ;
Jaaskelainen, Kari; (Vantaa, FI) ; Fabritius,
Marko; (Helsinki, FI) |
Correspondence
Address: |
BIRCH STEWART KOLASCH & BIRCH
PO BOX 747
FALLS CHURCH
VA
22040-0747
US
|
Family ID: |
8555905 |
Appl. No.: |
10/476684 |
Filed: |
December 8, 2003 |
PCT Filed: |
May 11, 2001 |
PCT NO: |
PCT/FI01/00459 |
Current U.S.
Class: |
110/347 ;
110/243 |
Current CPC
Class: |
F23C 10/002 20130101;
F23C 6/047 20130101; F23C 2201/101 20130101; F23C 2201/301
20130101 |
Class at
Publication: |
110/347 ;
110/243 |
International
Class: |
F23D 001/00; F23G
005/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 7, 2001 |
AU |
PR 7587 |
Claims
1. Combined fluidized bed and pulverized coal combustion method for
use in the combustion chamber (3) of a fluidized bed boiler, the
method comprising the steps of injecting fluidizing air (4) into a
fluidized bed (2) situated in the bottom portion of the combustion
chamber (3), feeding fuel from a first set of fuel feed means (5)
into the fluidized bed (2) and combusting the fuel in the fluidized
bed (2), feeding a mixture of pulverized coal and a carrier gas
from a second set of fuel feed means (6) into the combustion
chamber 3, to above the fluidized bed (2), into the combustion
chamber (3), to above the fluidized bed (2) is fed the mixture of
the pulverized coal and the carrier gas from the second set of fuel
feed means (6) at a mass flow rate ratio which is higher or at
least substantially equal to the upper ignition limit of the
mixture, and the mixture of the pulverized coal and the carrier gas
fed from the second set of fuel feed means (6) is diluted at least
by the fluidizing air (4), characterized in that at least a
fraction of the fuel fed via the second set of fuel feed means (6)
is combusted above the fluidized bed (2) at the temperature of 1300
to 1500.degree. C.
2. Method according to claim 1, characterized in that a fraction of
the pulverized coal fed via the second set of fuel feed means (6)
is combusted in the fluidized bed (2).
3. Method according to claim 1, characterized in that pulverized
coal with air is fed via the second set of fuel feed means (6) so
that the mass flow rate of these is from 1 to 10, most
advantageously from 3 to 7.
4. Method according to claim 1, characterized in that secondary air
(7) is injected into the combustion chamber (3), to above the
second set of fuel feed means (6).
5. Method according to claim 1, characterized in that tertiary air
(8) is injected into the combustion chamber (3), to above the level
of the secondary-air inlet means (7).
6. Method according to claim 1, characterized in that into the
combustion chamber (3) is fed via the second set of fuel feed (6)
means the mixture of fuel and carrier gas at a velocity of 20 to 30
m/s, most advantageously at a velocity of about 25 m/s.
7. Method according to claim 1, characterized in that at least
half, most advantageously 70 to 85%, of the pulverized coal fed via
the second set of fuel feed means (6) is combusted above the
fluidized bed (2).
8. Method according to claim 1, characterized in that into the
combustion chamber (3) is fed via the second set of fuel feed means
(6) the mixture of fuel and carrier gas at a level from 1 to 6 m,
most advantageously from 2 to 4 m, above the top level of the
fluidized bed (2).
9. Method according to claim 1, characterized in that the
pulverized coal fed via the second set of fuel feed means (6) is
combusted above the fluidized bed (2) at a ratio of 0.5 to 1
kg.sub.coal/kg.sub.air.
10. Combined fluidized bed and pulverized coal combustion system
comprising a combustion chamber (3) with an grid (13) adapted to
the bottom portion thereof, a fluidized bed (2) adapted to the
bottom portion of the combustion chamber (3) and formed by a
particulate matter bed material, air injection means adapted to the
grid (13) for feeding fluidizing air (44) into the bed material, a
first set of fuel feed means (5) for feeding fuel into the
fluidized bed (2), the system comprises a second set of fuel feed
means (6) for feeding a mixture of pulverized coal and a carrier
gas into the combustion chamber (3), at a distance above the
fluidized bed (2) at a mass flow rate ratio which is higher or at
least substantially equal to the upper ignition limit of the
mixture, and the second set of fuel feed means (6) are adapted at a
distance above the top level of the fluidized bed such that the
mixture of pulverized coal and a carrier gas discharged from said
means becomes dilutable with the fluidizing air (4) and at least in
partially combustible above the fluidized bed (2) characterized in
that the second set of fuel feed means (6) are arranged at such a
distance above the top level of the fluidized bed that the
combustion temperature of the pulverized coal above the bed is
1300-1500.degree. C.
11. System according to claim 10, characterized by means (7) for
injecting secondary air into the combustion chamber (3) at a level
above said second set of fuel feed means (6).
12. System for fluidized bed combustion according to claim 10,
characterized by means (8) for injecting tertiary air into the
combustion chamber (3) at a level above said secondary-air inlet
means (7).
13. System according to claim 10, characterized in that said second
set of fuel feed means comprise at least one duct (6) exiting into
the combustion chamber (3).
14. System according to claim 10, characterized in that at least
half, most advantageously 70 to 85% of the pulverized coal fed via
the second set of fuel feed means (6) is combustible above the
fluidized bed (2).
15. Method according to claim 2, characterized in that pulverized
coal with air is fed via the second set of fuel feed means (6) so
that the mass flow rate of these is from 1 to 10, most
advantageously from 3 to 7.
16. Method according to claim 4, characterized in that tertiary air
(8) is injected into the combustion chamber (3), to above the level
of the secondary-air inlet means (7).
17. Method according to claim 7, characterized in that the
pulverized coal fed via the second set of fuel feed means (6) is
combusted above the fluidized bed (2) at a ratio of 0.5 to
kg.sub.coal/kg.sub.air.
18. System for fluidized bed combustion according to aim 11,
characterized by means (8) for injecting tertiary air to the
combustion chamber (3) at a level above said secondary-r inlet
means (7).
Description
[0001] The invention relates to a combined fluidized bed and
pulverized coal combustion method according to the preamble of
claim 1, in which method fluidizing air is injected into a
fluidized bed residing in the bottom portion of a combustion
chamber, fuel is fed from a first set of fuel feed means into the
fluidized bed and is burnt in the fluidized bed, and the mixture of
pulverized coal and a carrier gas is fed from a second set of fuel
feed means into the combustion chamber, to above the fluidized
bed.
[0002] The invention also relates to a system for implementing the
method.
[0003] In firing by pulverized coal, the pulverized coal and the
combustion air are mixed with each other in a burner. To obtain an
efficient ignition and combustion of the mixture of coal and
combustion air introduced from a burner into the combustion chamber
of a boiler, these two must have a suitable fuel/air ratio. The
mixture does not ignite at all if the mass flow rate ratio of coal
to combustion air is below a certain lower ignition limit. In
firing by pulverized coal, the lower ignition limit is typically
about 0.2. When the fuel/air ratio is in the range 0.2 to 0.4, the
mixture will ignite but due to the lean mixture, the flame remains
unstable and the combustion temperature low. The generally used
fuel/air ratio in firing by pulverized coal is about 0.4, whereby
the flame is stable and the mixture burns at an elevated
temperature. A fuel/air ratio higher than this up to a certain
upper ignition limit gives good ignition but due to the rich
mixture renders a low combustion temperature. In firing by
pulverized coal, the upper ignition limit is typically about 1.0.
Mixtures richer than this cannot be ignited anymore.
[0004] In fluidized bed combustion, the fuel burns and becomes
partially gasified in a fluidized bed which resides above an air
distributor located in the bottom portion of the combustion chamber
of the boiler and is formed by particulate matter bed material and
the fuel mixed therewith. Conventionally, the bed material is sand.
The bed is maintained in a fluidized state by way of injecting
fluidizing gas, generally air, into the bed from nozzles located in
the air distributor. As the velocity of the fluidizing air is low
in the bed and a coarse particle size is selected for the bed
material, the fluidized bed is consequently formed in the bottom
portion of the combustion chamber. The solid fuel is generally fed
into the fluidized bed boiler via fuel feed nozzles adapted to the
walls of the combustion chamber. The combustion temperature in
fluidized bed combustion is typically about 800 to 950.degree.
C.
[0005] Due to the low combustion temperature and coarse milling of
the fuel, fluidized bed combustion of coal has given a relatively
low combustion efficiency as compared with many other firing
methods. The low combustion temperature also increases the amount
of nitrogen oxides formed in the combustion process. If a coarsely
milled fuel of high heat value, such as coal, is burnt in a
fluidized bed, accumulation of uncombusted fuel in the bottom
portion of the fluidized bed takes place, whereby the fuel burning
therein elevates the bed temperature and sintering of the bed
material occurs. To avoid this, the bed can be cooled by heat
exchangers located in the bed. However, the abrasive bed material
can rapidly corrode a heat exchanger embedded in the bed. The
amount of fuel accumulating in the bottom portion of the fluidized
bed can be reduced by way of moving the inlet point of the fuel to
above the fluidized bed and/or milling the fuel into a smaller
particulate size. The latter operation, however, generally dictates
the acquisition of a coal mill of a higher milling efficiency.
[0006] In U.S. Pat. No. 4,993,332 is described a hybrid combustion
system that combines fluidized bed combustion with pulverized coal
combustion, in which system the fluidized bed of a fluidized bed
boiler is fired in a conventional manner by coal complemented with
firing pulverized coal above the fluidized bed by means of a burner
mounted on the boiler wall. The object of this arrangement is to
reduce the disadvantages of fluidized bed combustion and pulverized
coal combustion.
[0007] When a conventional fluidized bed boiler is to be converted
suitable for hybrid combustion, the boiler need to be retrofitted
with a pulverized coal burner and an efficient coal mill in order
to provide the boiler with a coal feed of sufficiently fine and
consistent particle size. However, the new burner and efficient
coal mill represent a significant cost-increasing factor in
retrofitting a hybrid combustion system.
[0008] It is an object of the invention to provide an entirely
novel fluidized bed combustion method and system capable of
improving the combustion efficiency of fluidized bed combustion and
reducing nitrogen oxide emissions of fluidized bed combustion.
[0009] In an embodiment according to the invention, the fuel is fed
in a conventional manner into the fluidized bed of a fluidized bed
boiler and is combusted in the bed.
[0010] Additionally, to above the fluidized bed is admitted
pulverized coal through, e.g., a duct adapted to the wall of the
combustion chamber, at such a high coal/carrier gas ratio that the
fuel will not ignite in the close vicinity of the feed point. The
fluidizing air blown upward from the bottom of the bed dilutes the
mixture of the pulverized coal and the carrier gas thus allowing
the coal particles to ignite and burn in a rich flame pattern above
the fluidized bed. While a fraction of the coal can burn above the
fluidized bed, the other fraction falls into the fluidized bed and
is combusted therein.
[0011] More specifically, the fluidized bed combustion method
according to the invention is characterized by what is stated in
the characterizing part of claim 1.
[0012] Furthermore, the fluidized bed combustion system according
to the invention is characterized by what is stated in the
characterizing part of claim 11.
[0013] The invention offers significant benefits.
[0014] While the system according to the invention operates
generally in the same fashion as the hybrid combustion system
described above, but instead, the pulverized coal burner of the
prior-art system is replaced by a duct exiting into the combustion
chamber. Moreover, herein the fuel to be discharged from the duct
need not be milled to such a fineness and homogeneous particle size
as in pulverized coal combustion utilizing a burner, thus allowing
the system according to the invention to use a simple and
cost-efficient coal mill.
[0015] A major fraction of the coal fed above the fluidized bed is
burnt before the coal falls onto the fluidized bed. A certain
fraction of the coal is combusted in the bed, whereby the bed
temperature rises slightly thus contributing to the combustion
efficiency of the fluidized bed combustion system. However, the bed
temperature will not rise so high as to necessitate the use of
separate heat exchangers for cooling the bed. The coal combusted
above the fluidized bed burns under fuel-rich conditions at a high
temperature, whereby above the fluidized bed is created a so-called
reburn zone wherein nitrogen oxides formed in the bed are reduced
into molecular nitrogen. If a refuse fuel is combusted in the
fluidized bed, the combustion process of the bed forms dioxins and
other hazardous compounds that also are destroyed at the elevated
temperature of the reburn zone. Furthermore, the high temperature
of the reburn zone promotes the combustion of coal particles above
the fluidized bed, thus improving the combustion efficiency of the
boiler.
[0016] In the following, the invention is examined in more detail
by way of referring to the attached drawings, wherein
[0017] FIG. 1 is a schematic diagram of an embodiment of a combined
fluidized bed and pulverized coal combustion system according to
the invention.
[0018] FIG. 2 is a schematic diagram of another embodiment of a
combined fluidized bed and pulverized coal combustion system
according to the invention.
[0019] FIG. 3 is a diagram illustrating the amount of nitrogen
oxide emissions and the combustion efficiency as a function of
combustion temperature in the boiler.
[0020] A system illustrated in the drawing comprises a fluidized
bed boiler 1 having a fluidized bed 2 of particulate matter
situated in the bottom portion of the boiler combustion chamber 3.
A grid 13 disposed at the bottom of the combustion chamber 3
includes air feed means for admitting fluidizing air 4 into the bed
material. The velocity of the fluidizing air 4 is adjusted to keep
the fluidized bed 2 formed in the bottom portion of the combustion
chamber 3 in order to avoid a substantial loss of the bed material
from the bed along with the gas flow, thus generally disposing with
the need for circulating the bed material particles back to the bed
2. This type of fluidized bed is also known as a bubbling fluidized
bed. Although a fraction of the bed material particles can rise
with the gas flow up to the middle portion of the combustion
chamber 3, a bubbling fluidized bed 2 has a clearly discernible top
level. The fluidizing air 4 may also be used as the primary
combustion air in the combustion chamber 3. Coarse-milled fuel is
fed into the fluidized bed 2 in a conventional manner via a first
set of fuel feed means 5, such as one or more openings made to the
wall of the combustion chamber 3, whereto the fuel is transported
by a conveyor from a silo 16. Conventionally, the fuel is
coarse-milled coal, peat, biofuel, refuse fuel or a mixture of
these.
[0021] Into the combustion chamber 3 is also fed pulverized coal
via a second set of fuel feed means located above the fluidized bed
2, such as a duct 6 exiting into the combustion chamber 3. In its
simplest form, the duct 6 can be a pipe exiting into the combustion
chamber with a diameter of 150-300 mm. Also a multiple number of
ducts 6 can be used. The coal is ground in a coal mill 9 and the
coal comminuted into pulverized form is transported pneumatically
via a cyclone 10 to a coal storage 11, wherefrom it is moved with
the help of a screw conveyor 14, for instance, to the duct 6. The
carrier gas of the pulverized coal is pressurized by means of a
compressor 15, thus effecting the pneumatic discharge of the
pulverized coal from the duct 6 into the combustion chamber 3.
Advantageously, the carrier gas is air. Alternatively, the carrier
gas may be flue gas from the boiler 1, steam or nitrogen. The coal
discharged from the duct 6 need not be milled as fine and
homogeneous as is required for the fuel fed to a pulverized coal
burner thus allowing the system according to the invention to
operate utilizing an extremely simple type of coal mill 9.
[0022] At the entry of the fuel from the duct 6 to the combustion
chamber 3, the ratio of the mass flow rate of the pulverized coal
to the carrier gas is greater or at least substantially equal to
the upper ignition limit. The ratio of the mass flow rate of the
pulverized coal to the carrier gas is adjusted to a desired value
by means of controlling the rotational speeds of the screw conveyor
14 and the compressor 15. Advantageously, a 50-60% fraction of the
pulverized coal has a particle size smaller than 74 .mu.m. If the
carrier gas is air and the fuel is pulverized coal with a coarse
particle size of 70-150 .mu.m, the ratio of the mass flow rate of
the fuel to the carrier gas air is advantageously 1-10, most
advantageously 3-7. The velocity of the pulverized coal and the
carrier gas exiting from the duct 6 into the combustion chamber 3
is advantageously 20-30 m/s, most advantageously about 25 m/s.
Secondary combustion air is admitted into the combustion chamber 3
at the level of the duct 6 or thereabove via secondary-air inlet
means 7.
[0023] The feed point of the mixture of the carrier gas and the
pulverized coal is arranged depending on the size of the boiler 1
advantageously at a distance of 1 to 6 m, most advantageously 2 to
4 m, upward from the top level of the fluidized bed 2. The feed
point can be adapted below the secondary-air inlet point 7 or at
the same level with the secondary air inlet point 7. Typically, the
height of the fluidized bed 2 is about 1 m, whereby the feed point
of the carrier gas and the pulverized coal is about 2 to 7 m, most
advantageously 3 to 5 m above the grid 13. The fluidizing air 4
dilutes the mixture fed from the duct 6 thus allowing the particles
of the pulverized coal to ignite and burn under fuel-rich
conditions at a high temperature above the fluidized bed 2. The
ratio in this combustion process is typically 0.5 to 1
kg.sub.coal/kg.sub.air and the combustion temperature is 1300 to
1500.degree. C. The temperature is about 200 to 300.degree. C.
higher than the gas temperature in the freeboard above the
fluidized bed of a conventional fluidized bed boiler. Thus, the
temperature and other conditions in the space above the fluidized
bed 2 are adjusted to an optimal range (cf. FIG. 3), whereby the
formation of nitrogen oxides in combustion is minimized.
[0024] Advantageously, at least half, most advantageously 70 to
85%, of the pulverized coal fed via the duct 6 is combusted above
the fluidized bed 2 and the rest descends into the bed 2 so as to
undergo complete combustion in the bed 2. Resultingly, the
temperature of the fluidized bed 2 rises slightly, which improves
the combustion efficiency of the fuel fed from the first fuel feed
means 5 into the fluidized bed 2. However, the temperature of the
bed 2 does not herein rise excessively so that the bed material
would begin to sinter. If the distance of the duct 6 from the top
level of the fluidized bed 2 is made too small, a greater number of
coal particles can fall into the fluidized bed 2, whereby the
temperature of the fluidized bed 2 begins to rise. In contrast, if
the distance of the duct 6 from the top level of the fluidized bed
2 is made too large, coal particles have enough time to burn down
to a too small size before they reach the fluidized bed 2, whereby
they are conveyed out from the combustion chamber 3 along with the
flue gases, which is detrimental to the combustion efficiency of
the boiler 1.
[0025] Due to the high temperature and low oxygen content, above
the fluidized bed 2 is created a so-called reburn zone, wherein the
hydrocarbon radicals stemming from the combustion of the fuel
convert nitrogen oxides formed in the fluidized bed 2 into hydrogen
cyanide. Simultaneously, dioxins and other organic compounds formed
in the fluidized bed 2 from combustion of refuse fuel are
destroyed.
[0026] To above the secondary-air inlet means 7, into the wall of
the combustion chamber 3, are adapted tertiary-air inlet means 8
for injecting tertiary air into the combustion chamber 3. Herein,
any uncombusted fuel still existing in the flue gas flow is
combusted completely and hydrogen cyanide molecules formed in the
reburn reaction are converted into molecular nitrogen. Fine coal
particles, which are separated in the cyclone 10 from the flow of
millded coal being transported to the coal storage 1, are fed along
with the tertiary air into the combustion chamber 3, whereby the
above-mentioned reburn reaction takes place also at the level of
the tertiary air inlet means 8 as these fines are combusted.
Calcium carbonate or limestone taken from a container 12 is mixed
with the tertiary air 8 and the secondary air 7 in order to
eliminate sulfur compounds from the effluents.
[0027] Conventionally a fraction of the total fuel power of the
boiler 1 is fed as pulverized coal via the second set of fuel feed
means 6, while the other fraction is fed in coarse-milled form via
the first set of fuel feed means 5. The ratio of the fed fuel
powers can be varied over a wide range. The first set of fuel feed
means 5 and the second set of fuel feed means 6 may also be used
independently from each other. Herein, the entire fuel power of the
boiler 1 can be fed via the second set of fuel feed means 6, while
the first set of fuel feed means 5 are closed, and vice versa.
However, usually 10-90% of the total fuel power of the boiler 1 is
fed to above the fluidised bed 2 via the second set of fuel feeding
means 6.
[0028] The boiler 1 may also be operated as shown in FIG. 2,
whereby the mixture of fuel and carrier gas normally passed to the
second set of fuel feed means 6 is also discharged in to the
combustion chamber 3 via nozzles 17 opening into the fluidized bed
2. Herein, the nozzles 17 perform as the first set of fuel feed
means. A need to feed fuel via the nozzle 17 into the fluidized bed
2 arises, for instance, when the fuel supply from the silo 16 is
insufficient or the silo 16 is entirely nonoperative. Then, the
fuel discharged via the nozzle 17 serves as the preheating energy
source of the fluidized bed 2.
* * * * *